Cycle current from 0 to total-1.
iex> Kaffe.PartitionSelector.round_robin(nil, 3) 0
iex> Kaffe.PartitionSelector.round_robin(0, 3) 1
iex> Kaffe.PartitionSelector.round_robin(1, 3) 2
iex> Kaffe.PartitionSelector.round_robin(2, 3) 0